Fluorinated polymers containing sulfonic acid ion exchange groups, due to their ion conducting properties, have found widespread use in the manufacture of electrolyte membranes for electrochemical devices such as electrolysis cells and fuel cells. Notable examples are for instance proton exchange membrane (PEM) fuel cells which employ hydrogen as the fuel and oxygen or air as the oxidant.
In a typical PEM fuel cell, hydrogen is introduced into the anode portion, where hydrogen reacts and separates into protons and electrons. The membrane transports the protons to the cathode portion, while allowing a current of electrons to flow through an external circuit to the cathode portion to provide power. Oxygen is introduced into the cathode portion and reacts with the protons and electrons to form water and heat.
The membrane requires excellent ion conductivity, gas barrier properties (to avoid the direct mixing of hydrogen and oxygen), mechanical strength and chemical, electrochemical and thermal stability at the operating conditions of the cell. In particular, long-term stability of the membrane is a critical requirement: the lifetime goal for stationary fuel cell applications being up to 40,000 hours of operations, 20,000 hours of operation being the requirement for automotive fuel cell applications.
Attack of the proton exchange membrane by hydrogen peroxide radicals (′OH, ′OOH), which are generated during fuel cell operation, has often been described as one of the causes of membrane degradation. Radical degradation of the membrane contributes to the reduction of the life of service of the fuel cell. It is generally believed that, among other mechanisms, hydrogen peroxide is formed as a result of the reaction between hydrogen and oxygen that permeate through the membrane. Hydrogen peroxide then decomposes to form peroxy and hydroperoxy radicals, see for instance SCHLICK, S., et al. Degradation of fuel cell membranes using ESR methods: ex situ and in situ experiments. Polymer Preprints. 2009, vol. 50, no. 2, p. 745-746. Direct formation of the radicals is also believed to be possible.
Several attempts have been made to reduce radical degradation of fluorinated proton exchange membranes, for instance by incorporation into the membrane of suitable metallic salts or oxides. The use of salts of various metals, including rare earth metals, Al and Mn to increase the stability of ion exchange membranes for use in fuel cells is disclosed among others in EP 1702378 A (BDF IP HOLDINGS LTD) Sep. 20, 2006 and EP 1662595 A (TOYOTA CHUO KENKYUSHO) May 31, 2006.
US 20070213209 A (E.I. DU PONT DE NEMOURS) Sep. 13, 2007 discloses compounds for decomposing hydrogen peroxide in a fuel cell membrane electrode assembly which comprise a metal oxide from the group of alumina, silica, titanium oxides, zirconium oxide, manganese dioxide, Y2O3, Fe2O3, FeO, tin oxide, copper oxide, nickel oxide, tungsten oxide, germanium oxide, cerium oxides; a stabilizer selected from the group of metal ions and metalloid ions (e.g. boron); and at least one catalyst different from the stabilizer and selected from the group of cerium and ruthenium. The compounds disclosed in US20070213209 are prepared by adsorption of the catalyst on the metal oxide previously modified by the stabiliser. The catalyst particles are thus not incorporated into the crystal lattice of the metal oxide and may thus leach into the membrane and subsequently out of the membrane during the fuel cell operation.
ZHAO, D., et al. MnO2/SiO2-SO3H nanocomposite as hydrogen peroxide scavenger for durability improvement in proton exchange membranes. J. Membrane Science. 2010, vol. 346, p. 143-151. discloses nanosized mixed MnO2/SiO2 oxides having organic sulfonic acid groups grafted on their surface. The compounds are prepared by precipitating SiO2 on the surface of nanosized MnO2 followed by reacting the surface hydroxyl groups of SiO2 with suitable organic sulfonating reagents, such as cyclic sultonic acid esters. In the mixed MnO2/SiO2 oxide disclosed in Zhao et al. MnO2 is only physically combined with SiO2, this may lead to the reduction of Mn(IV) to Mn(II) during the fuel cell operation and, given the higher solubility of Mn(II) species, to their subsequent removal.
GILL, C. S., et al. Sulfonic acid-functionalized silica-coated magnetic nanoparticle catalysts. J. Catalysis. 2007, vol. 251, p. 145-152. discloses hybrid organic/inorganic catalysts comprising organic sulfonic acids grafted onto silica-coated magnetic nanoparticle supports.
In both of the systems described above organic hydrogenated moieties anchor the —SO3H groups to the SiO2 surface. The presence of these hydrogenated organic moieties in the inorganic oxide is believed to render the system poorly suitable for use in a fuel cell as it may provide an additional source of radical generation or radical degradation in the membrane under the fuel cell highly oxidising operating conditions.